We're also serious about this eventually being a viable open-source business, in the mold of WordPress.
The way it looks now, you're not engaging in open-source in the same way as WordPress does.
The GPL is enough, you don't need a contributor license agreement. What it actually means is that you are keeping an exit door if you ever want to close the source again in the future: contributors agree to license you their copyright for whatever future happens.
Probably lots of open-source projects use this method, but it sure is not reassuring. Please don't compare yourself again to WordPress, though. Nuance matters.

Occasionally, startups will ask me for advice. That's a shame, because I am a terrible person to ask for advice. The conversation usually goes something like this: We'd love to get your expert advice on our thing. I probably don't use your thing. Even if I tried your thing out and I gave you ...